1. Field of Invention
This invention is directed to iron-based mixed powders for use in metallurgy.
2. Description of Related Art
Iron-based mixed powders for use in powder metallurgy (hereinafter also referred to simply as xe2x80x9ciron-based mixed powderxe2x80x9d) are manufactured, generally, by adding: (1) an iron powder for an iron-based powder as a substrate material (which can be a mixture of one or more kinds of iron powder); (2).alloying powder(s) (one or more kinds of alloying powder, such as a copper powder, graphite powder, iron phosphide powder); optionally, (3) a lubricant such as zinc stearate (which can be a mixture of one or more kinds of lubricant): and, optionally, (4) machinability improving powder(s) (one or more kinds of machinability improving powder).
However, the iron-based mixed powder described has a problem that the starting powder, particularly, the alloying powder(s) tends to cause segregation. This is because the iron-based mixed powder contains plural kinds of powder of different sizes, shape and density. Specifically, the distribution of starting powders in the iron-based mixed powder is not uniform during transportation after mixing, charging to a hopper, discharging from the hopper or upon charging to the mold or during pressing.
For example, it is well-known for the mixed powder of the iron powder and the graphite powder that the iron powder and the graphite powder move and displace independently of each other in a transportation container due to vibration during track transportation and, as a result, the graphite powder of lower specific gravity floats to the surface and causes segregation. Further, because the mixed powder of the iron powder and the graphite powder charged in the hopper segregates due to movement in the hopper, it is also well-known that the concentration of the graphite powder is different, for example, between each of the initial stage, the middle stage and the final stage, of discharging from the hopper.
When the segregated iron-based mixed powder is charged in a mold and pressed into a molding product and the molding product is finally sintered into a sintered body as a final product, the composition fluctuates for each product (sintered products). As a result of the fluctuation of the composition, the size and the strength of products vary greatly to cause failed products.
Further, because each of the alloying powders to be mixed, such as copper powder, graphite powder and iron phosphide powder is finer than the iron-based powder, the specific surface area of the iron-based mixed powder increases by the mixing of the alloying powder(s) to lower the fluidity of the iron-based mixed powder. Lowering the fluidity of the iron-based mixed powder lowers the charging rate of the iron-based mixed powder into the mold and, therefore, lowers the production speed of the molding product (also referred to as compact powder or green compact).
As the countermeasure for such problems in the iron-based mixed powder, particularly, as a technique for preventing segregation, Japanese Patent Laid-Open No. 219101/1989, for example, discloses an iron powder for use in powder metallurgy, comprising from 0.3 to 1.3% of a lubricant, from 0.1 to 10% of an alloying element powder and the balance of an iron powder, in which the alloying element powder is adhered on the surface of the iron powder. According to this publication, the iron powder for use in powder metallurgy causes no segregation of the ingredients during handling and enables to obtain homogeneous sintered products. Japanese Patent Laid-Open No. 219101/1989 discloses zinc stearate and lithium stearate as an example of the lubricant.
In Japanese Patent Laid-Open No. 162502/1991, the present inventors previously proposed a method of manufacturing an iron-based mixed powder for use in powder metallurgy with less segregation of additives and less aging change for the fluidity. The method described in Japanese Patent Laid-Open No. 162502/1991 comprises conducting primary mixing by adding a fatty acid to an iron-based powder, then conducting secondary mixing by adding a metal soap to the alloying powder(s), elevating temperature during or after the secondary mixing, and then applying cooling during tertiary mixing, thereby adhering the alloying powder(s) to the surface of the iron-based powder by a bonding effect of a co-molten product of the fatty acid and the metal soap.
Further, Japanese Patent Publication No. 3004800 discloses an iron-based mixed powder using a binder not containing a metal compound as a binder for the alloying powder(s) to the surface of the iron-based powder. It is described that contamination of a sintering furnace can be reduced by the use of the binder material not containing the metal compound.
However, the iron-based mixed powder applied with the segregation-preventive treatment by each of the publications described above involves a problem in the die filling property to a mold and, particularly, has a property that the amount of charge to a narrow width portion of the mold (thin-walled cavity) tends to be decreased. In view of the above, the present inventors have experimentally confirmed the die filling property of the iron-based mixed powder applied with the segregation-preventive treatment by the publications described above. First, the result of this experiment is explained.
2 mass % of a copper powder and 0.8 mass % of a graphite powder were mixed with an atomized iron powder as the iron-based powder as the alloying powder(s), and 0.4 parts by weight of zinc stearate and 0.2 parts by weight of machine oil (spindle oil) as the binder were mixed based on 100 parts by weight of the total amount for the iron powder and the alloying powder(s), and heated to adhere the alloying powder(s) to the surface of the iron powder (example of a binder treatment). Then, 0.3 parts by weight of zinc stearate was mixed with them as a free lubricant. An iron-based mixed powder as a mixture of an iron powder and a free lubricant in which alloying powder(s) is adhered on the surface of the iron powder (existent product) was obtained by this treatment. 150 g of the iron-based mixed powder was charged in a shoe box sized 100 mmxc3x9720 mmxc3x9760 mm, as shown in FIG. 1.
The shoe box was moved in the direction to a mold at a speed of 200 mm/sec, stood stationary just above the mold for 1 second and then retracted to the original position in the arrangement as shown in FIG. 1. The iron-based mixed powder was charged into the mold by the operation. The mold used has a cavity and a thickness of T mm, length L of 60 mm and depth D of 60 mm. The thickness T mm was varied as 1, 2 and 5 mm.
After charging, the iron-based mixed powder charged in the cavity was molded at a pressure of 488 MPa and the weight of the obtained molding product was measured. Then, the charged density (=molding product weight/mold volume) was calculated to evaluate the die filling property of the iron-based mixed powder to the mold. The result for the iron-based mixed powder (known product) is shown in FIG. 2. It can be seen from FIG. 2 that the charged density decreases as the cavity thickness T of the mold decreases in the known product. For example, when the cavity thickness T of the mold is 1 mm, the known iron-based mixed powder is charged by less than one-half of the apparent density. As described above, when the cavity thickness of the mold is thin, die filling property of the iron-based mixed powder treated for segregation by the related art is deteriorated.
In the known product having low die filling property as described above, when it is charged into a mold, for example, of a gear shape, the charged density is lower at a narrow width portion of the tooth tip as compared with other portions. Then, when it is pressurized into the molding product and further sintered, because the amount of shrinkage differs depending on the portions, the dimensional accuracy of the part is deteriorated. Generally, when the charged density is different and the green density is different in different portions, the rate of dimensional change upon sintering also differs and, further, the sintering density is also different. Accordingly, in the portion at the tooth tip of the gear of low charged density, the sintering density tends to be lowered and, thus, the strength is lowered. Because maximum stress usually exerts on the portion of the tooth tip in the gear, it is required that the portion of the tooth tip has a higher strength and, preferably, the charged density is higher.
In view of the problems described above, Japanese Patent Laid-Open No. 267195/1997 discloses, for example, a powder charging method comprising disposing a pipe having a gas releasing holes at the surface in a shoe box, fluidizing a powder by the gas exiting from the gas releasing holes, and then charging the powder gravitationally into the cavity. However, because the technique described in Japanese Patent Laid-Open No. 267195/1997 requires a special apparatus, it has the problems of increasing the installation cost and increasing the manufacturing cost.
Further, in the field of sintered parts for use in automobiles, for instance, reduction of size for sintered parts has been desired along with a demand for a weight reduction of car bodies in recent years. However, stress exerted on parts tends to be increased along with the size reduction of the parts. Accordingly, for parts of an identical composition, those parts of higher strength, namely, those parts of higher density are desired (for the sintered product of an identical composition, the strength is generally increased as the density is increased). In order to obtain a sintered part of a reduced size and having high density, it is necessary that the iron-based mixed powder is applied with the segregation-preventive treatment and be excellent in compressibility. In addition, it is required for an iron-based mixed powder that it is excellent in die filling property to the narrower width portion of the mold, as well as that it has the characteristics described above.
This invention can advantageously overcome the problems in the related art described above and provide an iron-based mixed powder capable of manufacturing sintered parts of consistently high density and with less fluctuation of characteristics. Specifically, the invention can provide an iron-based mixed powder applied with a segregation-preventive treatment and excellent in the compressibility (high density for the molding product) and excellent in die filling property.
The present inventors have made an earnest study, in order to solve the foregoing problems, of various factors affecting the compressibility and the die filling property of the iron-based mixed powder applied with the segregation-preventive treatment (for example, a binder treatment).
For obtaining a high sintered density required generally for sintered parts, an atomized iron powder excellent in compressibility and fluidity of the mixed powder has usually been used as the iron-based powder. However, according to the study of the present inventors, it has been found that the iron-based mixed powder using the atomized iron powder as the iron-based powder is poor in die filling property to a mold having a narrow cavity compared with the iron-based mixed powder using a reduced iron powder. It is well known that mixed powder including reduced iron powder is inferior to that using atomized iron powder, not only in compressibility, but also in fluidity (measured by flow rate). Accordingly, it is an unexpected result that the mixed powder using the reduced iron powder shows high die filling property. However, it is difficult to obtain sufficient compressibility in the iron-based mixed powder using the reduced iron powder.
In view of the above, the present inventors further made a study on the reasons why the mixed powder using the reduced iron powder shows a high die filling property. Then, as a result of a further study noting that the distribution of the particle size is different between the reduced iron powder and the atomized iron powder, it has been found that the particle size distribution of the iron-based powder significantly affects the die filling property of the mixed powder.
Then, the present inventors have discovered that the die filling property can be improved remarkably in a case of using the atomized iron powder alone, or in a case of using an iron-based powder mainly comprising an atomized iron powder mixed with a reduced iron powder by forming an iron-based mixed powder using an iron-based powder having a predetermined particle size distribution, which is more restricted than that of conventional atomized iron powder. On the other hand, the present inventors have also discovered that the compressibility and the die filling property can be compatibilized by ensuring that the apparent density of the atomized iron powder and the iron-based mixed powder are more than a predetermined value. The present inventors have further discovered that use of appropriate binder and lubricant can also contribute to the further improvement of the die filling property. By the application of such discoveries, the present inventors have successfully obtained an iron-based mixed powder excellent in compressibility and remarkably improved in its die filling property.
FIG. 2 shows the die filling property of an iron-based mixed powder according to this invention as the product of the invention. The iron-based mixed powder according to this invention (inventive product) can be charged well even for a cavity having a thickness of 1 mm, and it can be seen that the die filling property is remarkably improved as compared with the known product.
This invention has been accomplished based on the findings described above and as a result of further study.
That is, this invention provides an iron-based mixed powder for use in powder metallurgy having an apparent density of at least about 3.1 Mg/m3, which comprises an iron-based powder, alloying powder(s), a binder and, optionally, machinability improving powder(s) and, preferably, further containing a free lubricant. The alloying powder(s) and, optionally, the machinability improving powder(s) are adhered by the binder to the surface of the iron-based powder (or applied with a binder treatment for adhesion). The iron-based powder is an atomized iron powder or a mixed powder of an atomized iron powder and the reduced iron powder, and with a maximum particle size of less than about 180 xcexcm, and with a particle size distribution containing 18.5 mass % or less of particles with a particle size of less than about 45 xcexcm, 46 mass % or more of particles with a particle size of from about 75 xcexcm to about 150 xcexcm, and less than 10 mass % of particles with a particle size of from about 150 xcexcm to about 180 xcexcm, and the apparent density of the atomized iron powder is at least about 2.85 Mg/m3. 
Further, in this invention, the content of the binder is preferably from about 0.1 parts by weight to about 1.0 parts by weight based on 100 parts by weight of the total amount for the iron-based powder, alloying powder(s) and the machinability improving powder(s).
Further, in this invention, the binder is preferably one or more members selected from stearic acid, oleamide, stearamide, a melted mixture of stearamide and ethylenbis(stearamide) and ethylenbis(stearamide).
Further, in this invention, the binder may comprise one or more members selected from oleic acid, spindle oil and turbine oil, and zinc stearate.
Further, in this invention, the content of the free lubricant is preferably from about 0.1 parts by weight to about 0.5 parts by weight based on 100 parts by weight of the total amount for the iron-based powder, the alloying powder(s) and the machinability improving powder(s).
Furthermore, in this invention, the free lubricant preferably contains one or more members selected from a thermoplastic resin powder, zinc stearate and lithium stearate, or, optionally, contains one or more members selected from stearic acid, oleamide, stearamide, a melted mixture of stearamide and ethylenbis(stearamide), ethylenbis(stearamide), polyethylene with a molecular weight of about 10,000 or less, and a melted mixture of ethylenbis(stearamide) and polyethylene with a molecular weight of about 10,000 or less.
Further in this invention, the thermoplastic resin powder preferably comprises at least about 50 mass %, based on the thermoplastic powder, of at least one member selected from acrylic esters, methacrylic esters and the aromatic vinyl compounds as a monomer polymerized therewith, and has a average primary particle size of from about 0.03 xcexcm to about 5.0 xcexcm, an average agglomeration particle size of from about 5 xcexcm to about 50 xcexcm, and an average molecular weight, measured by a solution specific viscosity method, of from about 30,000 to about 5,000,000.